Method for reporting channel information based on link adaptation in wireless local area network and the apparatus for the same

ABSTRACT

A method and a device for reporting a modulation and coding scheme (MCS) feedback in a wireless local area network are provided. A responding station receives, from a requesting station, a requesting Physical layer Protocol Data Unit (PPDU) for requesting a MCS feedback via a plurality of spatial streams. A recommended MCS is estimated under an assumption that the requesting station will transmit at least one first spatial stream among the plurality of spatial streams used for the requesting PPDU, The responding station transmits, to the requesting station, the MCS feedback including a recommended MCS field indicating the recommended MCS and a recommended stream field indicating a number of at least one recommended spatial stream. A number of the at least one first spatial stream used for estimating the recommended MCS is equal to the number of the at least one recommended spatial stream.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of co-pending U.S. patent applicationSer. No. 14/297,483 filed on Jun. 5, 2014, which is a Continuation ofU.S. patent application Ser. No. 13/500,352 filed on Apr. 5, 2012 (nowU.S. Pat. No. 8,787,341 issued on Jul. 22, 2014), which is filed as theNational Phase of PCT/KR2011/008993 filed on Nov. 23, 2011, which claimsthe benefit under 35 U.S.C. §119(e) to U.S. Provisional Application No.61/417,284 filed on Nov. 26, 2010, all of which are hereby expresslyincorporated by reference into the present application.

BACKGROUND OF THE INVENTION

The present invention relates to wireless communication and, moreparticularly, to a channel information reporting method based on a linkadaptation method performed between stations (STAs) in a Wireless LocalArea Network (WLAN) system and an apparatus for supporting the same.

With the advancement of information communication technologies, variouswireless communication technologies have recently been developed. Amongthe wireless communication technologies, a wireless local area network(WLAN) is a technology whereby Internet access is possible in a wirelessfashion in homes or businesses or in a region providing a specificservice by using a portable terminal such as a personal digitalassistant (PDA), a laptop computer, a portable multimedia player (PMP),etc.

The IEEE 802.11n is a technical standard relatively recently introducedto overcome a limited data rate which has been considered as a drawbackin the WLAN. The IEEE 802.11n is devised to increase network speed andreliability and to extend an operational distance of a wireless network.More specifically, the IEEE 802.11n supports a high throughput (HT),i.e., a data processing rate of up to above 540 Mbps, and is based on amultiple input and multiple output (MIMO) technique which uses multipleantennas in both a transmitter and a receiver to minimize a transmissionerror and to optimize a data rate.

With the widespread use of the WLAN and the diversification ofapplications using the WLAN, there is a recent demand for a new WLANsystem to support a higher throughput than a data processing ratesupported by the IEEE 802.11n. A next-generation WLAN system supportinga very high throughput (VHT) is a next version of the IEEE 802.11n WLANsystem, and is one of IEEE 802.11 WLAN systems which have recently beenproposed to support a data processing rate of above 1 Gbps in a MACservice access point (SAP).

The next-generation WLAN system supports the transmission of aMulti-User Multiple Input Multiple Output (MU-MIMO) scheme in which aplurality of non-AP STAs accesses a radio channel at the same time inorder to efficiently use the radio channel. According to the MU-MIMOtransmission scheme, an AP can transmit a frame to one or moreMIMO-paired STAs at the same time.

The AP and the plurality of MU-MIMO paired STAs may have differentcapabilities. In this case, a supportable bandwidth, modulation codingscheme (MCS), forward error correction (FEC), etc., may vary dependingon an STA type, usage, channel environment, etc.

According to the MU-MIMO transmission scheme, a transmitter can transmitdata to each of a plurality of MU-MIMO paired receivers through at leastone or more spatial streams. Herein, a channel between the transmitterand a first receiver and a channel between the transmitter and a secondreceiver may generate mutual interference. As such, the inter-channelinterference between the transmitter and the receiver may obstructcorrect data transmission and reception, which may result in decrease inoverall throughput of the WLAN system. Accordingly, when data istransmitted by using the MU-MIMO transmission scheme to improvethroughput of the WLAN system supporting the MU-MIMO transmissionscheme, there is a need to feed back a modulation and coding scheme(MCS) in sequence by considering interference between differentchannels.

Meanwhile, an environment for transmission and reception between an APand an STA may be changed. For example, the AP may want to control thenumber of spatial streams to be transmitted to the STA and send thespatial streams to the STA. Furthermore, the STA may determine that thetransmission and reception of data will be optimized by using whatspatial streams from among all the spatial streams available between theAP and the STA without being limited to spatial streams allocated by theAP. In this WLAN environment, there is a need for a link adaptationmethod in which the STA may feedback recommended spatial streams,determined by the STA, and a recommended a Modulation Coding Scheme(MCS) to be applied to the relevant spatial streams to the AP.

SUMMARY OF THE INVENTION

The present invention provides a method for reporting channelinformation based on a link adaptation in a wireless local area network,which supports Multiple User-Multiple Input Multiple Output (MU-MIMO)transmission.

In an aspect, a method for reporting channel information in a wirelesslocal area network system is provided. The method includes receiving adata block for requesting a modulation and coding scheme (MCS) feedbackfrom a requesting station, the data block including a data field and astream indicator indicating a number of at least one spatial stream inthe data field; determining, the MCS feedback based on the data block;and, transmitting the MCS feedback to the requesting station, the MCSfeedback including a recommended MCS and a recommended stream indicatorindicating a number of at least one recommended spatial stream. Thenumber of the at least one recommended spatial stream in the MCSfeedback is equal or less than the number of the at least one spatialstream in the data block.

If the number of the at least one recommended spatial stream in the MCSfeedback is less than the number of the at least spatial stream in thedata block, the recommended MCS may be estimated under an assumptionthat the at least one recommended spatial stream is at least one firstspatial stream among the at least one spatial stream in the data block.

The MCS feedback may be determined based on a bandwidth used fortransmitting the data field and a coding scheme applied to the datafield.

The data block further may include a channel bandwidth indicatorindicating the bandwidth; and, a coding scheme indicator indicating thecoding scheme.

The data block may further comprise a MRQ indicator requesting the MCSfeedback.

The MRQ indicator may be implemented 1 bit field.

The data block may be a physical layer convergence procedure (PLCP)protocol data unit (PPDU).

The data field may comprise a physical service data unit (PSDU) and theMRQ indicator may be included in the PSDU.

In another aspect, a wireless apparatus is provided. The apparatusincludes a transceiver transmitting and receiving radio signal; and, aprocessor operationally coupled to the transceiver. The processor isconfigured for the step of: receiving a data block for requesting amodulation and coding scheme (MCS) feedback from a requesting station,the data block including a data field and a stream indicator indicatinga number of at least one spatial stream in the data field; determiningthe MCS feedback based on the data block; and, transmitting the MCSfeedback to the requesting station, the MCS feedback including arecommended MCS and a recommended stream indicator indicating a numberof at least one recommended spatial stream. The number of the at leastone recommended spatial stream in the MCS feedback is equal or less thanthe number of the at least one spatial stream in the data block.

An STA determines what spatial streams have been optimized for a currentWLAN environment regarding spatial streams being used by an AP. Next,the STA feeds back information, indicating spatial streams to be usedfor MU-MIMO transmission between the AP and the STA, and information,indicating an optimized MCS to be applied to the relevant spatialstreams, to the AP. That is, MFB information fed back by the STAincludes spatial stream indication information and recommended MCSinformation. In a WLAN environment in which the use or an occupationsituation of spatial streams may be changed, an AP may receive MCSfeedback (MFB) information from an STA and perform MU-MIMO transmissionby using spatial streams fed back thereto and a recommended MCS. At thetime of the MU-MIMO transmission, the AP may utilize spatial streamsmore efficiently as compared with the existing method and apply anoptimal MCS estimated and computed by the STA. Accordingly, the overallthroughput of a WLAN can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the configuration of a WLAN system to whichembodiments of the present invention may be applied.

FIG. 2 is a diagram showing the PHY architecture of a WLAN system whichis supported by IEEE 802.11.

FIG. 3 is a diagram showing an example of a PPDU format used in a WLANsystem.

FIG. 4 is a diagram showing an example of a link adaptation method.

FIG. 5 is a diagram showing another example of a PPDU format used in aWLAN system.

FIG. 6 is a diagram showing another example of a link adaptation method.

FIG. 7 is a block diagram showing a format of the NDPA frame.

FIG. 8 is a diagram showing still another example of a link adaptationmethod.

FIG. 9 is a diagram showing a link adaptation method according to anembodiment of the present invention.

FIG. 10 is a block diagram showing an example of the existing linkadaptation subfield format.

FIG. 11 is a diagram showing an example of a link adaptation subfieldaccording to an embodiment of the present invention.

FIG. 12 is a block diagram showing an example of a link adaptationsubfield according to an embodiment of the present invention.

FIG. 13 is a diagram showing an example of a WLAN system to which theembodiments of the present invention may be applied.

FIGS. 14 and 15 are diagrams showing examples of a link adaptationmethod according to an embodiment of the present invention.

FIGS. 16 to 18 are diagrams showing examples of indicating recommendedspatial streams and a recommended MCS according to an embodiment of thepresent invention.

FIG. 19 is a block diagram showing a wireless apparatus according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a diagram showing the configuration of a WLAN system to whichembodiments of the present invention may be applied.

Referring to FIG. 1, A WLAN system includes one or more Basic ServiceSet (BSSs). The BSS is a set of stations (STAs) which can communicatewith each other through successful synchronization. The BSS is not aconcept indicating a specific area

An infrastructure BSS includes one or more non-AP STAs STA1, STA2, STA3,STA4, and STAS, an AP (Access Point) providing distribution service, anda Distribution System (DS) connecting a plurality of APs. In theinfrastructure BSS, an AP manages the non-AP STAs of the BSS.

On the other hand, an Independent BSS (IBSS) is operated in an Ad-Hocmode. The IBSS does not have a centralized management entity forperforming a management function because it does not include an AP. Thatis, in the IBSS, non-AP STAs are managed in a distributed manner. In theIBSS, all STAs may be composed of mobile STAs. All the STAs form aself-contained network because they are not allowed to access the DS.

An STA is a certain functional medium, including Medium Access Control(MAC) and wireless-medium physical layer interface satisfying theInstitute of Electrical and Electronics Engineers (IEEE) 802.11standard. Hereinafter, the STA refers to both an AP and a non-AP STA.

A non-AP STA is an STA which is not an AP. The non-AP STA may also bereferred to as a mobile terminal, a wireless device, a wirelesstransmit/receive unit (WTRU), a user equipment (UE), a mobile station(MS), a mobile subscriber unit, or simply a user. For convenience ofexplanation, the non-AP STA will be hereinafter referred to the STA.

The AP is a functional entity for providing connection to the DS througha wireless medium for an STA associated with the AP. Althoughcommunication between STAs in an infrastructure BSS including the AP isperformed via the AP in principle, the STAs can perform directcommunication when a direct link is set up. The AP may also be referredto as a central controller, a base station (BS), a node-B, a basetransceiver system (BTS), a site controller, etc.

A plurality of infrastructure BSSs including the BSS shown in FIG. 1 canbe interconnected by the use of the DS. An extended service set (ESS) isa plurality of BSSs connected by the use of the DS. APs and/or STAsincluded in the ESS can communicate with each another. In the same ESS,an STA can move from one BSS to another BSS while performing seamlesscommunication.

In a WLAN system based on IEEE 802.11, a basic access mechanism of amedium access control (MAC) is a carrier sense multiple access withcollision avoidance (CSMA/CA) mechanism. The CSMA/CA mechanism is alsoreferred to as a distributed coordinate function (DCF) of the IEEE802.11 MAC, and basically employs a “listen before talk” accessmechanism. In this type of access mechanism, an AP and/or an STA sensesa wireless channel or medium before starting transmission. As a resultof sensing, if it is determined that the medium is in an idle status,frame transmission starts by using the medium. Otherwise, if it issensed that the medium is in an occupied status, the AP and/or the STAdoes not start its transmission but sets and waits for a delay durationfor medium access.

The CSMA/CA mechanism also includes virtual carrier sensing in additionto physical carrier sensing in which the AP and/or the STA directlysenses the medium. The virtual carrier sensing is designed to compensatefor a problem that can occur in medium access such as a hidden nodeproblem. For the virtual carrier sending, the MAC of the WLAN systemuses a network allocation vector (NAV). The NAV is a value transmittedby an AP and/or an STA, currently using the medium or having a right touse the medium, to anther AP or another STA to indicate a remaining timebefore the medium returns to an available state. Therefore, a value setto the NAV corresponds to a period reserved for the use of the medium byan AP and/or an STA transmitting a corresponding frame.

The IEEE 802.11 MAC protocol, together with a Distributed CoordinationFunction (DCF), provides a Hybrid Coordination Function (HCF) based on aPoint Coordination Function (PCF) of performing periodical polling byusing the DCF and a polling-based synchronous access method so that allreception APs or STAs or both can receive data packets. The HCF includescontention-based Enhanced Distributed Channel Access (EDCA) and HCFControlled Channel Access (HCCA) using a contention-free-based channelaccess scheme employing polling mechanism as access schemes used by aprovider in order to provide data packets to a plurality of users. TheHCF includes a medium access mechanism for improving Quality of Service(QoS) of a WLAN, and QoS data can be transmitted in both a ContentionPeriod (CP) and a Contention-Free Period (CFP).

FIG. 2 is a diagram showing the PHY architecture of a WLAN system whichis supported by IEEE 802.11.

The PHY architecture of IEEE 802.11 includes a PHY Layer ManagementEntity (PLME), a Physical Layer Convergence Procedure (PLCP) sublayer210, and a Physical Medium Dependent (PMD) sublayer 200. The PLMEprovides the management function of a physical layer in association witha MAC Layer Management Entity (MLME). The PLCP sublayer 210 transfers aMAC Protocol Data Unit (MPDU), received from a MAC sublayer 220, to thePMD sublayer 200 or transfers a frame, received from the PMD sublayer200, to the MAC sublayer 220 according to an instruction of a MAC layerbetween the MAC sublayer 220 and the PMD sublayer 200. The PMD sublayer200, as a PLCP sublayer, enables the transmission and reception of aphysical entity between two STAs through a radio medium. The MPDUtransmitted by the MAC sublayer 220 is referred to as a Physical ServiceData Unit (PSDU) in the PLCP sublayer 210. The MPDU is similar to thePSDU, but if an Aggregated MPDU (A-MPDU) in which a plurality of MPDUsis aggregated is transferred, each MPDU and each PSDU may be differentfrom each other.

In a process of transferring the PSDU, received from the MAC sublayer220, to the PMD sublayer 200, the PLCP sublayer 210 adds a supplementarysubfield, including information necessary for a physical transceiver, tothe PSDU. The field added to the PSDU may include tail bits necessary torestore a PLCP preamble, a PLCP header, and a convolution encoder to azero state. The PLCP sublayer 210 receives a TXVECTOR parameter,including control information necessary to generate and transmit aPhysical Layer Convergence Procedure (PLCP) Protocol Data Unit (PPDU)and control information necessary for a receiving STA to receive andinterpret the PPDU, from the MAC sublayer 220. The PLCP sublayer 210uses the information included in the TXVECTOR parameter in order togenerate the PPDU including the PSDU.

The PLCP preamble functions to enable a receiver to be prepared for asynchronization function and an antenna diversity before the PSDU istransmitted. A data field may include padding bits, a service fieldincluding a bit sequence for resetting a scrambler, and a coded sequencein which the bit sequence having tail bits added thereto has beenencoded in the PSDU. Here, an encoding scheme may be either a BinaryConvolutional Coding (BCC) encoding scheme or a Low Density Parity Check(LDPC) encoding scheme according to an encoding scheme supported by anSTA that receives a PPDU. The PLCP header includes a field includinginformation about a PLCP Protocol Data Unit (PPDU) to be transmitted.The PLCP header will be described in more detail later with reference toFIG. 3.

The PLCP sublayer 210 generates the PPDU by adding the field to the PSDUand transmits the generated PPDU to a receiving STA via the PMD sublayer200. The receiving STA receives the PPDU, obtains information necessaryto restore data from a PLCP preamble and a PLCP header, and restores thedata. The PLCP sublayer of the receiving STA transfers an RXVECTORparameter, including control information included in a PLCP preamble anda PLCP header, to an MAC sublayer so that the MAC sublayer can interpretthe PPDU and obtain data in a reception state.

A WLAN system supports transmit channels of a more contiguous 160 MHzband and a discontiguous 80+80 MHz band in order to support a higherthroughput. Furthermore, the WLAN system supports a MultipleUser-Multiple Input Multiple Output (MU-MIMO) transmission scheme. In aWLAN system supporting the MU-MIMO transmission scheme, an AP or an STAor both that try to transmit data may transmit data packets to one ormore MU-MIMO-paired receiving STAs at the same time.

Referring back to FIG. 1, in a WLAN system, such as that shown in FIG.1, the AP 10 may transmit data to an STA group including at least oneSTA, from among the plurality of STAs 21, 22, 23, 24, and 30 associatedtherewith, at the same time. An example where the AP performs MU-MIMOtransmission to the STAs is shown in FIG. 1. In a WLAN system supportingTunneled Direct Link Setup (TDLS), Direct Link Setup (DLS), or a meshnetwork, however, an STA trying to send data may send a PPDU to aplurality of STAs by using the MU-MIMO transmission scheme. An examplewhere an AP sends a PPDU to a plurality of STAs according to the MU-MIMOtransmission scheme is described below.

The data respectively transmitted to each of the STAs may be transmittedthrough different spatial streams. The data packet transmitted by the AP10 may be a PPDU, generated and transmitted by the physical layer of aWLAN system, or a data field included in the PPDU, and the data packetmay be referred to as a frame. That is, a data field included in a PPDUfor SU-MIMO or MU-MIMO or both may be called an MIMO packet. In anexample of the present invention, it is assumed that a targettransmission STA group MU-MIMO-paired with the AP 10 includes the STA 121, the STA 2 22, the STA 3 23, and the STA 4 24. Here, data may not betransmitted to a specific STA of the target transmission STA groupbecause spatial streams are not allocated to the specific STA.Meanwhile, it is assumed that the STAa 30 is associated with the AP 10,but not included in the target transmission STA group.

In order to support MU-MIMO transmission in a WLAN system, an identifiermay be allocated to a target transmission STA group, and the identifiermay be called a group ID. An AP transmits a group ID management frame,including group definition information, to STAs supporting MU-MIMOtransmission in order to allocate a group ID to the STAs. The group IDis allocated to the STAs based on the group ID management frame prior toPPDU transmission. A plurality of group IDs may be allocated to one STA.

Table 1 below shows information elements included in the group IDmanagement frame.

TABLE 1 Order Information 1 Category 2 VHT action 3 Membership status 4Spatial stream position

The frames of the category field and the VHT action field correspond tomanagement frames. The category field and the VHT action field are setto identify that the relevant frames are group ID management frames usedin the next-generation WLAN system supporting MU-MIMO.

As in Table 1, group definition information includes the membershipstatus information, indicating whether an STA belongs to a specificgroup ID, and spatial stream position information indicating that whatplace is the spatial stream set of a relevant STA located from all thespatial streams according to MU-MIMO transmission if the STA belongs tothe relevant group ID.

Since a plurality of group IDs is managed by one AP, membership statusinformation provided to one STA needs to indicate whether the STAbelongs to each of the group IDs managed by the AP. Accordingly, themembership status information may exist in an array form of subfields,indicating whether the STA belongs to each group ID. The spatial streamposition information may exist in an array form of subfields, indicatinga position of a spatial stream set occupied by an STA regarding eachgroup ID, because the spatial stream position information indicates aposition for each group ID. Furthermore, the membership statusinformation and the spatial stream position information for one group IDmay be implemented within one subfield.

If an AP transmits a PPDU to a plurality of STAs according to theMU-MIMO transmission scheme, the AP includes information, indicating agroup ID, in the PPDU, and transmits the information as controlinformation. When an STA receives the PPDU, the STA checks whether it isa member STA of a target transmission STA group by checking a group IDfield. If the STA is checked to be a member of the target transmissionSTA group, the STA may check that what place is a spatial stream set,transmitted thereto, placed from all the spatial streams. Since the PPDUincludes information about the number of spatial streams allocated to areception STA, the STA can receive data by searching for spatial streamsallocated thereto.

FIG. 3 is a diagram showing an example of a PPDU format used in a WLANsystem.

Referring to FIG. 3, a PPDU 300 may include an L-STF 310, an L-LTF 320,an L-SIG field 330, a VHT-SIG Afield 340, a VHT-STF 350, a VHT-LTF 360,a VHT-SIG B field 370, and a data field 380.

The PLCP sublayer forming the physical layer converts a PSDU, receivedfrom the MAC layer, into the data field 380 by adding necessaryinformation to the PSDU, generates the PPDU 300 by adding fields, suchas the L-STF 310, the L-LTF 320, the L-SIG field 330, the VHT-SIG Afield 340, the VHT-STF 350, the VHT-LTF 360, and the VHT-SIGB field 370,to the data field 380, and transmits the PPDU 300 to one or more STAsthrough the PMD sublayer forming the physical layer. Control informationnecessary for the PLCP sublayer to generate the PPDU and controlinformation, included in the PPDU and transmitted so that a receivingSTA can use the control information to interpret the PPDU, are providedfrom the TXVECTOR parameter received from the MAC layer.

The L-STF 310 is used for frame timing acquisition, Automatic GainControl (AGC) convergence, coarse frequency acquisition, etc.

The L-LTF 320 is used for channel estimation for demodulating the L-SIGfield 330 and the VHT-SIG A field 340.

The L-SIG field 330 is used for an L-STA to receive the PPDU 300 andobtain data by interpreting the PPDU 300. The L-SIG field 330 includes arate subfield, a length subfield, parity bits, and a tail field. Therate subfield is set to a value indicating a bit rate for data to betransmitted now.

The length subfield is set to a value indicating the octet length of aPhysical Service Data Unit (PSDU) that the MAC layer requests a physicallayer to send the PSDU. Here, an L_LENGTH parameter related toinformation about the octet length of the PSDU is determined on thebasis of a TXTIME parameter related to transmission time. TXTIMEindicates a transmission time determined by the physical layer in orderto transmit a PPDU including the PSDU, in response to a transmissiontime that the MAC layer has requested the physical layer to send thePSDU. Since the L_LENGTH parameter is a parameter related to time, thelength subfield included in the L-SIG field 330 includes informationrelated to the transmission time.

The VHT-SIG A field 340 includes control information (or signalinformation) necessary for STAs, receiving the PPDU 300, to interpretthe PPDU 300. The VHT-SIG A field 340 is transmitted through two OFDMsymbols. Accordingly, the VHT-SIG A field 340 may be divided into aVHT-SIG A1 field and a VHT-SIG A2 field. The VHT-SIG A1 field includeschannel bandwidth information used for PPDU transmission, informationindicating whether Space Time Block Coding (STBC) is used, informationindicating a scheme for transmitting a PPDU, from among SU and MU-MIMOschemes, information indicating a target transmission STA groupincluding a plurality of STAs that are MU-MIMO-paired with an AP if thetransmission scheme is the MU-MIMO scheme, and information about spatialstreams allocated to each STA of the target transmission STA group. TheVHT-SIG A2 field includes short Guard Interval (GI)-related information.

The information indicating the MIMO transmission scheme and theinformation indicating the target transmission STA group may beimplemented into a piece of MIMO indication information. For example,they may be implemented in the form of a group ID. The group ID may beset to a value having a specific range. A specific value of the rangemay indicate the SU-MIMO transmission scheme, and the remaining valuesof the range may be used as an identifier for a relevant targettransmission STA group if the PPDU 300 is transmitted according to theMU-MIMO transmission scheme.

If the group ID indicates that the PPDU 300 is transmitted according tothe SU-MIMO transmission scheme, the VHT-SIG A2 field includes codingindication information, indicating whether a coding scheme applied to adata field is a Binary Convolution Coding (BCC) scheme or a Low DensityParity Check (LDPC) coding scheme, and Modulation Coding Scheme (MCS)information about a channel between a sender and a recipient.Furthermore, the VHT-SIG A2 field may include the AID of an STA to whichthe PPDU 300 will be transmitted or a partial AID including some bitsequences of the AID or both.

If the group ID indicates that the PPDU 300 is transmitted according tothe MU-MIMO transmission scheme, the VHT-SIG A field 340 includes codingindication information indicating whether a coding scheme applied to adata field to be transmitted to reception STAs that are MU-MIMO-pairedis the BCC scheme or the LDPC coding scheme. In this case, ModulationCoding Scheme (MCS) information for each reception STA may be includedin the VHT-SIG B field 370.

The VHT-STF 350 is used to improve AGC estimation performance in MIMOtransmission.

The VHT-LTF 360 is used for an STA to estimate a MIMO channel. TheVHT-LTF 360 may be set to the number corresponding to the number ofspatial streams through which the PPDU 300 is transmitted because thenext-generation WLAN system supports MU-MIMO. Additionally, full channelsounding is supported. If the full channel sounding is performed, thenumber of VHT-LTFs may be further increased.

The VHT-SIG B field 370 includes dedicated control information which isnecessary for a plurality of MIMO-paired STAs to obtain data byreceiving the PPDU 300. Accordingly, only when common controlinformation included in the VHT-SIG B field 370 indicates that thereceived PPDU 300 has been subjected to MU-MIMO transmission, an STA maybe designed to decode the VHT-SIG B field 370. On the other hand, if thecommon control information indicates that the received PPDU 300 is for asingle STA (including SU-MIMO), an STA may be implemented not to decodethe VHT-SIG B field 370.

The VHT-SIG B field 370 includes information about an MCS andinformation about rate matching for each STA. The VHT-SIG B field 370further includes information indicating the length of a PSDU which isincluded in a data field for each STA. The information indicating thelength of the PSDU is information indicating the length of the bitsequence of the PSDU and may be indicated by an octet unit. The size ofthe VHT-SIG B field 370 may vary an MIMO transmission type (MU-MIMO orSU-MIMO) and a channel bandwidth used for PPDU transmission.

The data field 380 includes data intended to be transmitted to an STA.The data field 380 includes a service field for resetting a PLCP ServiceData Unit (PSDU) to which an MAC Protocol Data Unit (MPDU) in the MAClayer has been transferred and a scrambler, a tail field including a bitsequence necessary to restore a convolution encoder to a zero state, andpadding bits for normalizing the length of a data field.

In a WLAN system, such as that shown in FIG. 1, if the AP 10 intends totransmit data to the STA 1 21, the STA 2 22, and the STA 3 23, the AP 10may transmit the PPDU to an STA group including the STA 1 21, the STA 222, the STA 3 23, and the STA 4 24. In this case, the data may betransmitted in such a manner that spatial streams are not allocated tothe STA 4 24 and a specific number of spatial streams are allocated toeach of the STA 1 21, the STA 2 22, and the STA 3 23, as in FIG. 2. Inthe example of FIG. 2, it can be seen that one spatial stream has beenallocated to the STA 1 21, three spatial streams have been allocated tothe STA 2 22, and two spatial streams have been allocated to the STA 323.

In a WLAN system, in order to efficiently use a given channel, theMU-MIMO transmission scheme must be able to be used by schedulingseveral STAs at the same time. MU-MIMO transmission means that data canbe transmitted to a plurality of STAs at the same time. In this case, anoverall throughput of a system can be improved. Here, if each STA feedsback optimal MCS information, including spatial streams information, toan AP in a time-sequential manner, system performance may be furtherimproved.

In data transmission and reception processes in a WLAN system, currentopen-loop link adaptation using an acknowledgement (ACK) frame, anon-acknowledgement (NACK) frame, etc. is disadvantageous in thatchannel information between an AP and an STA is not sufficientlyutilized. In order to improve the throughput of a WLAN system bysupplementing the disadvantage, a method of supporting closed-loop linkadaptation for feeding back more accurate channel information to an APmay be taken into consideration.

In a WLAN system, for channel sounding, a Null Data Packet Announcement(NDPA) frame—a Null Data Packet (NDP) transmission method is supported.According to the NDPA frame—NDP transmission method, a plurality ofMU-MIMO-paired STAs may estimate channels on the basis of the NDP.Furthermore, the STAs may estimate channels on the basis of the LTF of acommon PPDU, including data, as well as the NDP frame. A link adaptationmethod based on the NDPA frame—NDP transmission method and a linkadaptation method based on a common PPDU may be proposed by applying theestimation to link adaptation method.

In below description of a plurality of link adaptation methods accordingto the embodiments of the present invention, it is assumed that an APtransmitting by using MU-MIMO scheme is a MCS feedback (MFB) requesterand STAs MIMO paired with the AP are MFB responders. However, thepresent invention is not limited thereto, and thus the MFB requester andthe MFB responder may be the AP and/or the STAs.

FIG. 4 is a diagram showing an example of a link adaptation method.

There is proposed the link adaptation method based on a PPDU includingdata. An AP may obtain a beamforming matrix suitable for transmission toeach STA by using information obtained through a previous channelsounding procedure. Accordingly, the AP may send the PPDU of a precodedform in which an MCS Request (MRQ) has been set to each ofMU-MIMO-paired STAs.

For link adaptation, each STA must estimate a channel. The estimationmay be performed on the basis of a VHT-LTF which is the PLCP preamble ofthe transmitted PPDU. Since data units are aggregated and transmitted indata transmission employing a PPDU, the STA may check the reception ofthe received PPDU by using a Block Ack (BA) frame. The STA may includeMCS Feedback (MFB) information in the BA frame and transmit the BAframe.

In order to request an MCS from each STA, an AP may set the MCS request(MRQ) subfield of an MCS request or Antenna Selection (ASEL) Indication(MAI) field, included in the MAC header of a PSDU transmitted to theSTA, to 1. In this case, since a Null Data Packet (NDP) is not used, anNDP announcement subfield is set to 0. This setting may be implementedby using the HT control field of the MAC header which forms the PSDUtransmitted to each STA. Furthermore, the setting of the HT controlfield may be implemented by setting the HT control field included in acontrol wrapper frame. In other words, the setting of the HT controlfield may be implemented by including the control frame in the controlwrapper frame and setting the HT control field within the controlwrapper frame because the HT control field may not be included in acommon control frame supported by a WLAN system.

Each STA may include MFB information, related to the result of channelestimation, in the HT control field of the control wrapper frameincluding the BA frame and transmit the HT control field to the AP.

In the example shown in FIG. 4, if an AP transmits a PPDU having aformat, such as that shown in FIG. 5, to STAs, the group ID of a VHT-SIGA field includes information about that the AP is participating in datatransmission with what STAs. It is assumed that an STA1, an STA2, anSTA3, and an STA4 are included in a target STA group and three spatialstreams, two spatial streams, one spatial stream, and one spatial streamhave been allocated to the STA1, the STA2, the STA3, and the STA4,respectively.

Although a total of 7 spatial streams have been used, 8 LTFs arerequired in order for a receiver to perform channel estimation. Inlegacy preambles L-STF and L-LTF and a VHT-SIG A field for a legacy STA,one spatial stream is transmitted through all transmission antennas.Fields subsequent to a VHT-STF field are subject to Cyclic Shift Delay(CSD) and precoding according to a precoding matrix and are thentransmitted.

Each of the STAs that have received the PPDU obtains the data througheach decoding process and transmits a BA frame to the AP. Each STAincludes estimation MCS information, assuming that the data is receivedthrough spatial streams allocated to the relevant STA using channelinformation estimated based on the LTF, in the BA frame and sends the BAframe to the AP. When the MCS is estimated, the relevant STA mayestimate and compute the MCS by taking interference that may be causedby spatial streams not allocated to the relevant STA into consideration.

According to the setting of a group ID, The STA1 includes an MCSFeedback (MFB) in the BA frame and sends the BA frame to the AR When theAP sends a Block ACK Request (BAR) to a specific STA in responsethereto, the specific STA may include MFB information in a BA frame andsend the BA frame to the AP.

Additionally, whenever the BAR is successively transmitted to eachSTASTA, a corresponding STA may send MFB information simultaneously witha BA frame. Likewise, in the process of estimating and computing theMCS, the corresponding STA may estimate and compute the MCS by usingspatial streams allocated thereto, assuming that the remaining spatialstreams may function as interference.

FIG. 6 is a diagram showing another example of a link adaptation method.

Referring to FIG. 6, an AP sends an NDP to each of STAs. The NDP is aframe that is used by each STA for channel estimation. The NDP has aformat of a PPDU not including a data field. As in the reception of thePPDU, when the NDP is received, each of the STAs may perform channelestimation on the basis of an LTF included in the NDP. Before sendingthe NDP, the AP sends an NDPA frame, (i.e., one of a plurality ofcontrol frames), in order to inform that the NDP will be transmitted.

FIG. 7 is a block diagram showing a format of the NDPA frame.

Referring to FIG. 7, the NDPA frame 700 includes a frame control field710, a duration field 720, a Receiver Address (RA) field 730, aTransmitter Address (TA) field 740, a sounding sequence field 750, atleast one STA information field 760, and a Frame Check Sequence (FCS)field 770. The frame control field 710 includes control informationrelated to the NDPA frame 700. The duration field 720 indicates thelength of the NDPA frame 700. The RA field 730 indicates an address ofan STA that receives the NDPA frame 700 and may indicate a broadcastaddress. The TA field 740 indicates an address of an AP or an STA orboth which transmit the NDPA frame 700. The sounding sequence field 750includes the number of sounding sequences to be now included. The STAinformation field 760 includes information to identify a target channelsounding STA and information about feedback information according tochannel sounding. If one or more target channel sounding STAs exist, oneor more STA information fields 760 may be included in the NDPA frame700.

Referring back to FIG. 6, the NDPA frame is a control frame used for achannel sounding procedure according to the NDPA frame—NDP transmissionmethod. Accordingly, since the NDPA frame itself does not include an HTcontrol field, information to request MCS estimation cannot be includedin the NDPA frame. However, since an STA can acquires information forthe MCS estimation based on the NDPA frame and performs channelestimation using the NDP, a link adaptation method based on the NDPAframe and the NDP is also possible. To this end, there is proposed amethod of including the NDPA frame in a control wrapper frame andtransmitting the control wrapper frame so that an MRQ can be triggeredby sending the NDPA frame. The MRQ setting may be implemented by settingthe HT control field included in the control wrapper frame.

An MRQ and NDP announcement are set in the HT control field of thecontrol wrapper frame including the NDPA frame. Furthermore, informationabout the NDP transmitted to each STA must be included in the HT controlfield. The group ID of the VHT-SIG A field of a PPDU includesinformation about a target MU-MIMO transmission STA group by the AP.Each STA may obtain indication information about the number of spatialstream from a unicasted and/or broadcasted PPDU.

The setting of the MRQ and the NDP announcement may be implemented bysetting the link adaptation subfield and the NDP announcement subfieldof the HT control field.

Meanwhile, the NDP itself includes a VHT-SIG A field includinginformation about a group ID and the allocation of spatial streams.However, the group ID of the NDP is agreed to indicate that atransmission scheme is the SU transmission scheme. Accordingly,information that may be obtained from the VHT-SIG A field of the NDP isinformation about the allocation of spatial streams and the MCS.

Meanwhile, as described above, the MCS-related information applied toeach spatial stream may be differently implemented according to a valueset in a group ID. Accordingly, in case of an NDP, the MCS-relatedinformation should be set as in MU transmission irrespective of thegroup ID. In this case, an STA may obtain the MCS-related information byinterpreting a VHT-SIG B field.

An STA may know that it is engaged with transmission with an AP based onthe unicasted or broadcasted PPDU. If information about the number ofspatial streams of the PPDU is equal to information about the number ofspatial streams of the NDP, the STA may obtain information about thenumber of spatial streams allocated thereto. Furthermore, the STA mayobtain MCS information, applied to each of the spatial streams, from theVHT-SIG B field of the NDP. Each STA may estimate and compute an optimalMCS by taking spatial streams, allocated to other STAs, intoconsideration on the basis of the MCS information and send MFBinformation to an AP according to the polling scheme of the AP. Each STAsends a feedback frame including an HT control field, including theobtained spatial streams information and the MCS information. The linkadaptation method according to the above description is based on theNDPA frame—NDP transmission method. Accordingly, the feedback frame maybe a VHT beamforming compressed frame, and the feedback frame mayfurther include Channel State Information (CSI) according to channelsounding.

The AP receives the CSI and all pieces of possible MFB information fromthe STAs through the link adaptation procedure. Each STA indicates thatthe MFB information is an MCS that has been optimally estimated for eachof spatial streams. When the MFB information is received from each STA,the AP may select a proper MCS by taking an optimal system throughputupon next transmission or fairness for every STA into consideration sothat spatial streams are fairly allocated to the highest degree.

In the example of FIG. 6, it is assumed that the number of spatialstreams that may be transmitted by the AP is 8 and the number of spatialstreams available for the AP is 4, 3, 2, and 1. The AP may do not acceptall the requests of the STAs. In this case, it is assumed that the MUtransmission scheme rather than the SU transmission scheme is determinedto be advantageous in terms of a system throughput and spatial streamsneed to be fairly allocated to each STA.

Meanwhile, when the STAs feed back MCSs after estimating the MCSs, eachSTA may feed back the MCS on the basis of the SU-MIMO transmissionscheme and, at the same time, may feed back the MCS assuming that atransmission scheme is an MU-MIMO transmission scheme. Here, theassumption of the MU-MIMO transmission scheme and the feedback of theMCS may be performed assuming that the number of spatial streams and atransmission power have other restrictions, unlike in the SU-MIMOtransmission scheme.

FIG. 8 is a diagram showing still another example of a link adaptationmethod. In this example, it is assumed that the number of STAs that areMU-MIMO-paired with an AP is 3 and there is no change of a channel.

Referring to FIG. 8, an AP transmits a PPDU to an STA 1, an STA 2, andan STA 3 according to the MU-MIMO transmission scheme at step S810. ThePPDU may be transmitted on the basis of the result that previous channelsounding has been performed.

Next, there is no data to be transmitted from the AP to the STA 3 or theconnection of the AP with the STA 3 may be terminated at step S820.

The AP may allocate additional radio resources, generated as a result ofthe termination of the connection with the STA 3, to the existing STA 1and STA 2. In this case, a new channel sounding procedure is notnecessary in calculating a new precoding matrix for reallocating theresources because there is no change in a channel.

The AP initiates a link adaptation procedure for the STA 1 and the STA2. The AP transmits a PPDU, including an HT control field in which anMRQ has been set, to the STA 1, the STA 2, and the STA 3 at step S830.

Each of the STA 1, the STA 2, and the STA 3 estimates a channel and anMCS on the basis of the VHT-LTF of the PPDU and feeds back MFBinformation to the AP at step S840. The MFB information feedback, by theSTAs MU-MIMO paired with the AP, is performed based on a pollingmechanism, transmitting and/or receiving a BA frame and/or BA request(BAR) frame.

The AP performs data transmission on the basis of a new MCS obtained inthe process at step S850.

In order to improve the throughput of the next-generation WLAN system, amore efficient link adaptation method needs to be proposed. When thereis a change in the number of spatial streams to be transmitted by an AP,a spatial stream indication scheme may be proposed. Furthermore, when anSTA determines to use the number of spatial streams smaller than thenumber of spatial streams received from an AP from a viewpoint of thethroughput, a link adaptation method of feeding back MFB information tothe AP is proposed.

In a WLAN system, a maximum of 4 spatial streams may be allocated toeach STA and transmitted according to a downlink MU-MIMO transmissionscheme. If the number of spatial streams to be transmitted from an AP toan STA is reduced, the AP may request MCS estimation for a situationfrom the STA without performing a new channel sounding procedure. The APmay initiate the link adaptation procedure by transmitting a PPDUincluding MRQ. The STA may also inform the AP that it will be efficientto use one or more spatial streams, corresponding to specific streams ofall special streams, by using information about the number of spatialstreams to be changed, received from the AP.

The number of spatial streams is not changed when a specific STA doesnot use all the spatial streams, but at least one recommended spatialstream may be indicated even when a Signal to Interference plus NoiseRatio (SINR) is changed in each spatial stream.

FIG. 9 is a diagram showing a link adaptation method according to anembodiment of the present invention.

An AP transmits a PPDU to an STA 1 and an STA 2 by using the MU-MIMOtransmission scheme at step S910.

If the AP detects that the number of spatial streams transmitted to eachSTA will be reduced at step S920, the AP may inform each STA of thenumber of spatial streams to be reduced. It is provided that APtransmits a PPDU including MRQ for a method for informing each STA ofthe number of spatial streams to be reduced (S930).

This method may be implemented by setting spatial stream indicationinformation of the VHT-SIG A1 field in the PPDU including data.

Each STA may determine MCS information, estimated based on the PPDU orNDP or both, and what spatial streams will be used and may feed MFBinformation back to the AP at step S940. MFB information feedback isperformed based on polling by BAR frame transmitted from AP. When theMFB information including spatial stream indication information and MCSindication information is fed back from each of the STAs, the APtransmits a PPDU on the basis of the MFB information at step S950.

Here, each STA may feed the relevant information back to the AP bysetting the link adaptation subfield of a control wrapper frameincluding a BA frame. The link adaptation subfield is included in an HTcontrol field. The setting of the link adaptation subfield is describedbelow.

FIG. 10 is a block diagram showing an example of the existing linkadaptation subfield format.

Referring to FIG. 10, a link adaptation subfield 1000 includes an HT/VHTsubfield 1010, a training request (TRQ) subfield 1020, an MCS request orantenna selection (ASEL) Indication (MAI) subfield 1030, an MCS Feedbacksequence identifier (MFSI) subfield 1040, and an MCS feedback andAntennal Selection command/data (MFB/ASELC) subfield 1050.

The HT/VHT subfield 1010 indicates whether the link adaptation subfield1000 or an HT control field including the link adaptation subfield 1000or both are for HT transmission supporting SU-MIMO or VHT transmissionsupporting SU/MU-MIMO.

The TRQ subfield 1020 indicates a request to send a training sequencefor initiating a channel sounding procedure.

The MAI subfield 1030 includes an MRQ subfield 1031 and an MRQ sequenceidentifier (MSI) subfield 1032. The MRQ subfield 1031 indicates whetherto initiate a link adaptation procedure. That is, the MRQ subfield 1031indicates whether MCS estimation has been requested. The MSI subfield1032 includes information to a sequence for identifying the request,when MCS estimation has been requested, identifying the specificrequest.

The MFSI subfield 1040 may be set to a value of the MSI value includedin a frame triggering MFB information.

The MFB/ASELC subfield 1050 includes an MCS subfield 1051 anda-Number-of-Spatial-Streams (N_(ss)) subfield 1052. The MCS subfield1051 indicates an MCS recommended by an STA. The N_(ss) subfield 1052indicates the number of spatial streams which is recommended by an STA.

A total of four bits may be necessary for an STA to inform an AP whatspatial streams will be preferably used because a maximum of 4 spatialstreams may be allocated to one STA. To this end, there is proposed amethod of utilizing the TRQ subfield 1020 and the N_(ss) subfield 1052.In a WLAN system supporting VHT transmission, a channel soundingprocedure is performed on the basis of the NDPA frame—NDP transmissionmethod. Thus, 1 bit for the TRQ subfield 1020 may be used for otherpurposes. Furthermore, a scheme in which an STA directly signalizes whatspatial streams will be used, instead of informing an AP of the numberof recommended spatial streams, is proposed below. For this scheme, 3bits of the existing N_(ss) subfield 1052 may be used.

FIG. 11 is a diagram showing an example of a link adaptation subfieldaccording to an embodiment of the present invention.

Referring to FIG. 11, a link adaptation subfield 1100 includes an HT/VHTsubfield 1110, an MAI subfield 1130, an MFSI subfield 1140, and anMFB/ASELC subfield 1150. Referring to FIG. 11(a) and FIG. 11(b), thelink adaptation subfield includes spatial stream subfields 1100 a, 1110b, and 1120 b of four bits in size, instead of the N_(ss) subfield 1052of the existing format. The spatial stream subfield may be implementedin a bitmap type that may inform whether to use each of spatial streamsin the form of a bit value. Unlike in FIG. 11(a), in FIG. 11(b), the TRQsubfield and the N_(ss) subfield of the existing format shown in FIG.11(a) are used as the spatial stream subfields. 1 bit that implementsthe TRQ subfield in FIG. 11(a) may be set to indicate whether a firstspatial stream SS0 is used (1110 b), and 3 bits that implements theN_(ss) subfield in FIG. 11(a) may be set to indicate whether second tofourth spatial streams SS1 to SS3 are used (1120 b).

For example, the AP may inform the STA 1 and the STA 2 that threereduced spatial streams and two reduced spatial streams may betransmitted to the STA 1 and the STA 2, respectively, while transmittingfour spatial streams to each of the STA 1 and the STA 2. Each of theSTAs may first use a spatial stream having a high Signal to Noise Ratio(SNR) or SINR value by using SNR or SINR information about each spatialstream, obtained from the result of the current transmission, in nexttransmission. Assuming that first, second, and fourth spatial streamshave high SNR ratios according to the estimation result of the STA 1,the STA 1 may feed values (1, 1, 0, 1) of spatial stream subfields backto the AR Furthermore, each STA may estimate an MCS on the basis of arelevant spatial stream and feed MCS information back to the AP.

The above method may also be applied to SU-MIMO transmission. An AP maytransmit data to an STA by using eight spatial streams. In this case,the size of a spatial stream subfield may be 8 bits. For a format of alink adaptation subfield according to this method, reference may be madeto FIG. 12. Referring to FIG. 12, a link adaptation subfield 1200includes an N_(ss) subfield 1210 and a spatial stream subfield 1220. TheN_(ss) subfield 1210 indicates the number of recommended spatialstreams, and the spatial stream subfield 1220 includes information of abitmap type indicating what spatial streams will be used.

Meanwhile, in a normal link adaptation procedure, STAs perform optimalMCS estimation by taking interference resulting from other STAs intoconsideration on the basis of the number of spatial streams transmittedby an AP. A series of processes, however, may not result in an optimalresult from a viewpoint of a system throughput. It would be better totransmit the number of spatial streams smaller than the number ofspatial streams designated by an AP by using a high MCS from a viewpointof an instantaneous system throughput. A system environment, such asthat shown in FIG. 13, is described below as an example.

FIG. 13 is a diagram showing an example of a WLAN system to which theembodiments of the present invention may be applied. An AP performsMU-MIMO transmission to K MU-MIMO-paired STAs. It is assumed that the APincludes N_(T) transmission antennas and all the STAs include N_(R)reception antennas.

A signal r_(k) received from a k^(th) STA is represented by Equation 1below.

$\begin{matrix}{r_{k} = {{H_{k}P_{k}s_{k}} + {H_{k}{\sum\limits_{\underset{i \neq k}{i = 1}}^{K}{P_{i}s_{i}}}} + n_{k}}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In Equation 1, P_(k) is the precoding matrix of the AP, s_(k) is a datastream to be transmitted to the k^(th) STA, and L is the number of datastreams. H_(k) indicates a channel between the AP and the k^(th) STA.The precoding matrix P_(k), the data stream s_(k), and the channelmatrix H_(k) may be represented by Equation 2 below.

P _(k) εC ^(N) ^(T) ^(×L) , s _(k) εC ^(L×1) , H _(k) εC ^(N) ^(R) ^(×N)^(T)   [Equation 2]

If each STA uses a reception matrix W_(k)εC^(L×N) ^(T) , apost-processing received signal ŝ_(k) may be represented by Equation 3below.

$\begin{matrix}{{\hat{s}}_{k} = {{W_{k}r_{k}} = {{W_{k}H_{k}P_{k}s_{k}} + {W_{k}\left( {{H_{k}{\sum\limits_{\underset{i \neq k}{i = 1}}^{K}{P_{i}s_{i}}}} + n_{k}} \right)}}}} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack\end{matrix}$

When a linear MMSE filter is used a receiving filter, the SINR of anl^(th) stream of the k^(th) STA may be represented by Equation 4 below.

SIN R _(k,j) =p _(k,l) ^(H) H _(k) ^(H) R _(K,l) ⁻¹ H _(k) p _(k,l)

R _(k,j) =H _(k) P _(k) P _(k) ^(H) H _(k) ^(H) +R _(n) _(k) −H _(k) p_(k,l) p _(k,l) ^(H) H _(k) ^(H)  [Equation 4]

In Equation 4, p_(k,l) refers to an l^(th) column of the precodingmatrix P_(k).

Instantaneous mutual information of a system may be represented byEquation 5 below.

$\begin{matrix}{I = {\sum\limits_{k = 1}^{K}{\sum\limits_{l = 1}^{L_{k}}{\log \left( {1 + {SINR}_{k,l}} \right)}}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

Instantaneous mutual information of the k^(th) STA may be represented byEquation 6 below.

$\begin{matrix}{{I_{k}(L)} = {{\sum\limits_{l = 1}^{L}{\log \left( {1 + {SINR}_{k,l}} \right)}} = {\log {\sum\limits_{l = 1}^{L}\left( {1 + {SINR}_{k,l}} \right)}}}} & \left\lbrack {{Equation}\mspace{14mu} 6} \right\rbrack\end{matrix}$

Assuming that the transmission power of the AP is constant based on theequation, if power is differently allocated to spatial streams, it wouldbe better for the k^(th) STA to use the number of spatial streamssmaller than a maximum number of available spatial streams from a betterinstantaneous throughput on the k^(th) STA. That is, I_(k)(L′)<I_(k)(L)may occur regarding L′<L. In this situation, a method of estimating andcomputing an MCS when data is transmitted using reduced spatial streamsis described below.

It is assumed that a code rate for an MCS index n is R(n) (where n=0, 1,. . . , 8). It is also assumed that an MCS index calculated on the basisof spatial streams transmitted from an STA to an AP is n and an MCSindex estimated on the basis of the reduced spatial streams is n′. Inthis case, there is proposed an MCS index computation method, such asEquation 7.

$\begin{matrix}{n^{\prime} = {\underset{m}{argmax}\left\{ m \middle| {{{I_{k}\left( L^{\prime} \right)} \cdot {R(m)}} \leq {{I_{k}(L)} \cdot {R(n)}}} \right\}}} & \left\lbrack {{Equation}\mspace{14mu} 7} \right\rbrack\end{matrix}$

In general, an AP may find it difficult to detect a change of thechannel between an STA and the AP. Since the AP requests MCS estimationthrough a PPDU that is subject to MU-MIMO transmission, MFB informationis determined on the basis of the PPDU. Here, if an STA may be able todetect a change, such as the number of optimized spatial streams thathas not been detected by an AP, a method of indicating such detectionthrough MFB information is proposed.

FIGS. 14 and 15 are diagrams showing examples of a link adaptationmethod according to an embodiment of the present invention.

Referring to FIG. 14, an AP transmits a PPDU in which an MRQ has beenset to an STA 1, an STA 2, and an STA 3 at step S1410. The STAstransmits respective BA frames to the AP at steps S1421, S1422, andS1423 The BA frame is transmitted through a control wrapper frame. MFBinformation is included in the control wrapper frame and then fed backto the AP. An STA that will perform a feedback in the first place isdetermined on the basis of group ID and spatial stream indicationinformation set by the AP, and subsequent feedback steps are performedwhen the AP transmits respective BAR frames at step S1431 and S1432.

Referring to FIG. 15, an AP transmits an NDPA frame in which an MRQ hasbeen set to an STA 1, an STA 2, and an STA 3 at step S1510 and thentransmits an NDP thereto at step S1520 In response thereto, the STA 1,the STA 2, and the STA 3 feed respective pieces of MFB information backto the AP at steps S1531, S1532, and S1533 The MFB information may beincluded in a feedback frame transmitted by each STA. The feedback framemay be a VHT compressed beamforming frame transmitted when a channelsounding response is made. The MFB information may be included in an HTcontrol field and then transmitted. An STA that will transmit thefeedback frame in the first place is determined on the basis of group IDand spatial stream indication information set by the AP. Subsequentfeedback steps are performed when the AP transmits respective pollframes at steps S1541 and S1542.

The MFB information may be estimated and computed on the basis of anRXVECTOR which is a reception information parameter for a PPDUtransmitted by an AP and triggers an MRQ. The RXVECTOR may include achannel bandwidth parameter, a group ID parameter, an N_(ss) parameter,a transmit chain number parameter, an applied encoding scheme parameter,a beamforming parameter, and an STBC parameter. The MFB subfield of anHT control field having a VHT format, included in a BA frame or afeedback frame transmitted by an STA, includes an N_(ss) subfieldindicating the number of spatial streams that is recommended by the STA.Here, the N_(ss) subfield may be set to a value equal to or smaller thanthe number of spatial streams which is allocated to the STA andindicated by the N_(ss) parameter of the reception information parameterRXVECTOR.

Assuming that a spatial stream dimension N to be estimated by an STA isgiven by an AP, if the STA arbitrarily feeds an MCS suitable for spatialstream dimension smaller than the spatial stream dimension N, there maybe a problem in that the AP does not know how the MCS will be used. Forexample, if an AP transmits a PPDU to the STA and makes the STA estimatechannel for two spatial streams by using column vectors v1 and v2 of aprecoding matrix applied to the PPDU, the STA may determine that itwould be advantageous to receive one spatial stream rather than toreceive the two spatial streams. The STA may include indicator for theone spatial stream and an MCS suitable for the one spatial stream in MFBinformation and feed the MFB information back to the AR In this case,the AP does not know how the one spatial stream will be best optimized.For this, there is proposed a method of indicating detailed spatialstream configuration information when an STA feeds the number of spatialstreams smaller than the number of spatial streams, allocated by an ARback as MFB information.

In a signal for channel estimation requesting an MCS, if the number ofspatial streams smaller than a set of given spatial streams is fed back,there is a proposed a method of feeding information indicating specificspatial streams to be used and a recommended MCS to be applied to thespecific spatial streams. To this end, there is proposed a method ofimplementing the spatial stream indication information in a bitmap typeand feeding back the spatial stream indication information.

For example, assuming that the total number of transmitted spatialstreams is 4, an MCS, indicating a higher throughput as a result ofcomparing a throughput estimated by computing an MCS when the 4 spatialstreams are used in an STA with a throughput estimated by computing anMCS when 3 spatial streams are used in the STA, may be fed back. Here,the most optimal spatial stream configuration may be indicated in abitmap type. An AP does not configure a new precoding column vector, butmay continue to use the existing precoding column vector and thusreceive a more optimal MCS from an STA. Accordingly, a process ofcomputing a new precoding column vector when an optimal spatial streamis used can be omitted, and an optimal MCS can be used between the APand the STA.

A detailed method of informing an AP of spatial streams and an MCSrecommended by an STA is described below with reference to FIGS. 16 to18. It is assumed that the total number of spatial streams supported forMIMO transmission in a WLAN system is 8. It is also assumed that each ofthe 8 spatial streams, allocated to each STA, can be distinguished fromeach other using index values. What spatial stream group among allspatial streams transmitted from AP is allocated to a specific STA issignaled by a spatial stream position information element of group IDmanagement frame. Therefore, the STA determine at least one spatialstream allocated to itself when AP transmits PPDU to the STA withsignaling a number of spatial streams allocated to each of STAs MU-MIMOpaired with the AP. If each of spatial streams transmitted from AP canbe distinguished from the others among the spatial streams by indicesallocated to the spatial streams, the STA may use and manage at leastone spatial stream allocated to itself by using the indices.

FIGS. 16 and 17 are diagrams showing an example of a spatial streamindication according to an embodiment of the present invention.

An AP transmits a frame, requesting MCS estimation, to an STA by using aspatial stream 1 to a spatial stream 4. The STA configures MFBinformation 1600, including spatial stream indication information 1610and recommended MCS indication information 1620. In the spatial streamindication information 1610, whether each of spatial streams is used maybe configured in a bitmap type by allocating 1 bit to each of thespatial streams. The spatial stream indication information 1610 may bedivided into a part 1611 for spatial streams allocated to the STA and apart 1612 for the remaining spatial streams not allocated to the STA. Inthe part 1611 for the allocated spatial streams, a bit for a recommendedspatial stream may be set to 1, and a bit for a non-recommended spatialstream may be set to 0. The part 1612 not allocated by the AP may bebasically set to 0.

If the first bit and the second bit of the bitmap are set to 1 and theremaining bits thereof are set to 0 as in FIG. 16, the AP may determinethat the STA has recommended the spatial stream 1 and the spatial stream2. Furthermore, when transmitting a PPDU to the STA, the AP may use thespatial stream 1 and the spatial stream 2 and use a recommended MCSindicated by the MCS indication information 1620.

If the first bit and the third bit of the bitmap are set to 1 and theremaining bits thereof are set to 0 as in FIG. 17, the AP may determinethat the STA has recommended the spatial stream 2 and the spatial stream3. Furthermore, when transmitting a PPDU to the STA, the AP may use thespatial stream 2 and the spatial stream 3 and use a recommended MCSindicated by the MCS indication information 1620.

When feeding back MFB information, as a bitmap type, the number ofspatial streams smaller than the number of spatial streams indicated ina signal transmitted for performing MRQ and channel estimation, an STAmust compute an optimal MCS and compare throughputs for all possiblecombinations. When an MCS is requested using eight spatial streams, anSTA must compute an optimal MCS and compare throughputs for 7 or lowerall spatial stream combinations. It may result in overload when the STAoperates. In order to avoid this problem, there is proposed a method ofrecommending the number of spatial streams smaller than the number ofspatial streams required in an MRQ, but previously agreeing aconfiguration of the recommended spatial streams between an AP and anSTA. An example of the proposed method may be given as shown n FIG. 18.

FIG. 18 is a diagram showing an example of spatial stream indicationaccording to an embodiment of the present invention.

MFB information 1800 configured by an STA includes N_(ss) indicationinformation 1810 and recommended MCS indication information 1820. Whenfeeding the MFB information back to an AP, the STA recommends that theAP use the number of spatial streams equal to or smaller than the numberof spatial streams, allocated to the STA, through a PPDU in which an MRQhas been triggered. In this case, it is assumed that spatial streamsamong the allocated spatial streams are contiguously used from a spatialstream having a smaller index value. The MFB information may beestimated and computed assuming that the relevant spatial streams willbe used.

In FIG. 18, since a spatial stream1 to a spatial stream 4 have beenallocated when the AP triggered the MRQ, The STA estimates and computingthe MFB information under an assumption that the spatial stream 1 andthe spatial stream 2 are used by AP and/or the STA. If, the PPDUtriggering the MRQ allocates the spatial stream 3 to spatial stream 6 tothe STA, the STA may estimate and computing the MFB information under anassumption that the spatial stream 3 and the spatial stream 4 are usedfor transmitting a PPDU by AP and/or the STA.

On receipt MFB information from the STA, the AP may determine to use thespatial stream/and the spatial stream2 through the recommended number ofspatial streams information included in the MFB information, and acquirethe MCS recommended by the STA.

FIG. 19 is a block diagram showing a wireless apparatus according to anembodiment of the present invention.

Referring to FIG. 19, a wireless apparatus 1900 includes a processor1910, a memory 1920, and a transceiver 1930. The transceiver 1930transmits and/or receives a radio signal, and implements an IEEE 802.11physical (PHY) layer. The processor 1910 functionally coupled to thetransceiver 1930 is configured to implement a MAC layer and/or a PHYlayer. The processor 1910 may be configured to generate and transmit thePPDU format according to the embodiments of the present invention. Theprocessor 1910 may further be configured to receive PPDU, interpret aplurality of fields in the PPDU, acquiring control information by theinterpreting and data by using the control information. The processor1910 may further be configured to estimate a channel and feed the MFBinformation back to the AP. The processor 1910 may be configured totransmit a PPDU according to the MFB information, on receipt the MFBinformation. The processor 1910 is configured for implementingembodiments for the present invention shown in FIG. 9 to FIG. 18.

The processor 1910 and/or the transceiver 1930 may include anapplication-specific integrated circuit (ASIC), a separate chipset, alogic circuit, and/or a data processing unit. When the embodiment of thepresent invention is implemented in software, the aforementioned methodscan be implemented with a module (i.e., process, function, etc.) forperforming the aforementioned functions. The module may be stored in thememory 1920 and may be performed by the processor 1910. The memory 1920may be located inside or outside the processor 1910, and may be coupledto the processor 1910 by using various well-known means.

What is claimed is:
 1. A method for reporting a modulation and codingscheme (MCS) feedback in a wireless local area network, the methodcomprising; receiving, by a responding station, from a requestingstation, a requesting Physical layer Protocol Data Unit (PPDU) forrequesting a MCS feedback via a plurality of spatial streams; estimatinga recommended MCS under an assumption that the requesting station willtransmit at least one first spatial stream among the plurality ofspatial streams used for the requesting PPDU; and transmitting, by theresponding station, to the requesting station, the MCS feedbackincluding a recommended MCS field indicating the recommended MCS and arecommended stream field indicating a number of at least one recommendedspatial stream, wherein a number of the at least one first spatialstream used for estimating the recommended MCS is equal to the number ofthe at least one recommended spatial stream.
 2. The method of claim 1,wherein the number of the at least one recommended spatial stream isless than a number of the plurality of spatial streams used for therequesting PPDU.
 3. The method of claim 1, wherein the requesting PPDUincludes an MCS request (MRQ) field that requests the responding stationto provide the MCS feedback.
 4. The method of claim 1, wherein therecommended stream field has 3 bits.
 5. The method of claim 1, whereinthe recommended MCS field has 4 bits.
 6. The method of claim 1, whereinthe requesting PPDU includes a stream indicator indicating the number ofthe plurality of spatial streams used for the requesting PPDU.
 7. Adevice configured to report a modulation and coding scheme (MCS)feedback in a wireless local area network, the device comprising: atransceiver configured to transmit and receive a radio signal; and, aprocessor operationally coupled to the transceiver and configured to:control the transceiver to receive, from a requesting station, arequesting Physical layer Protocol Data Unit (PPDU) for requesting a MCSfeedback via a plurality of spatial streams; estimate a recommended MCSunder an assumption that the requesting station will transmit at leastone first spatial stream among the plurality of spatial streams used forthe requesting PPDU; and control the transceiver to transmit, to therequesting station, the MCS feedback including a recommended MCS fieldindicating the recommended MCS and a recommended stream field indicatinga number of at least one recommended spatial stream, wherein a number ofthe at least one first spatial stream used for estimating therecommended MCS is equal to the number of the at least one recommendedspatial stream.
 8. The device of claim 7, wherein the number of the atleast one recommended spatial stream is less than a number of theplurality of spatial streams used for the requesting PPDU.
 9. The deviceof claim 7, wherein the requesting PPDU includes an MCS request (MRQ)field that requests the responding station to provide the MCS feedback.10. The device of claim 7, wherein the recommended stream field has 3bits.
 11. The device of claim 7, wherein the recommended MCS field has 4bits.
 12. The device of claim 7, wherein the requesting PPDU includes astream indicator indicating the number of the plurality of spatialstreams used for the requesting PPDU.